What’s weirder than a slick, oily substance suddenly appearing at a northwest Indiana beach on Lake Michigan? Its equally sudden disappearance.

Swimmers at Porter Beach at Indiana Dunes National Lakeshore came out of the water Monday afternoon to find themselves covered in an oily substance and there was a silver sheen on the lake, according to the Coast Guard and the Indiana Department of Environmental Management (IDEM).

But by the next day, it was gone.

"They checked the beach, and they can't find any evidence of it," IDEM spokesman Barry Sneed told ABCNews.com. "[Authorities] figure it may have sunk, or moved farther north. It's a strange phenomenon."

Preliminary tests indicate the slippery sludge included D-gluconic acid, a mild acid used to clean metals, and tricalcium orthophosphate, an additive found in food and fertilizers, said Dan Goldblatt, a spokesman for the Department of Environmental Management.

"Initial reports make it look like it's not something that's highly toxic," Goldblatt said. "Once we know more we can make a better determination."

Meanwhile, the Coast Guard's safety marine unit in Chicago has reviewed a week's worth of video showing vessels in the Port of Indiana, but have not seen anything spilling from the ships, according to Petty Officer 3rd Class Christopher Yaw.

Scientists who examined water samples said that substance had not killed any local organisms.

So where in the world did it come from, and where did it go? Final results should be ready by the end of the week, and tests from the U.S. Geological Survey are pending. Stay tuned.

NBC Chicago reports on the murky mystery in the video below:

Related Great Lakes stories on MNN:

Oily substance in Lake Michigan remains a mystery
Described as a 'strange phenomenon' by officials, the dark sheen appeared one day and was gone the next.